This invention generally relates to electric heating systems, and in particular to an impedance heating system that can be used in a wide range of applications to heat materials such as gases or fluids that are flowing through pipelines or contained in storage tanks. For example, the impedance heating system can be used to heat the contents of a rail car storage tank, or a pipeline assembly associated with a storage and distribution system having several miles of pipeline.
As is known in the art, the object to be heated in an impedance heating system generates heat as a result of an alternating current (AC) passing through it. In a pipeline impedance heating system, the walls of the pipe act as an electric heating element when AC voltage is applied to the pipeline. Impedance heating systems can be used to prevent freezing in cold weather, to maintain fluidity of substances that are viscous or solid at ambient temperature, to heat temperature sensitive materials, and to heat fluids and gases to a desired temperature. Substances heated by such a system can include, for example, asphalt, chocolate, chemicals, creams, crude oils, cooking oils, fuel, formaldehyde, gases, high pressure air, ink, lubricants, molasses, molten magnesium, paint, paraffin, propane gas, soaps, sulfur, synthetic resins, syrups, tars, wax and water.
In a typical configuration, a low voltage current (e.g., 80 V or less) is applied from a transformer associated with a power supply to one end of an electrically conductive pipe. Current flows through the pipe to the other remote end, and then back to the transformer via a return path. In pipeline storage and distribution assemblies, the return path typically includes an insulated electrical return cable disposed parallel to the pipeline. In storage vessel applications, the return path includes an outer conductive member (such as an outer pipe) in which the inner member is disposed. In this configuration, the outer pipe typically is electrically connected to the inner pipe at the remote end.
Impedance heating systems are capable of producing substantial heat within a storage tank or pipeline. Resistance (I.sup.2 R) heating develops when current flows in the pipe. The effective resistance of the pipe varies based upon pipe length, composition and wall thickness. If the material transported by the pipe is electrically conductive, the resistive load further increases. The rapid changes of a 60 Hz alternating current induce an electromotive force and self inductance that opposes current flow (reactance). The reactance combines with the resistance to further impede current flow and generate heat. Magnetic flux coupling between current paths in the impedance heating system also produces heat due to hysteresis (molecular friction) and eddy currents.
Use of impedance heating systems in rail car applications often have significant drawbacks. During switching, coupling and uncoupling of the rail cars, the impedance heating system is subjected to a tremendous shock loads. The impedance heaters experience further vibrational forces during rail transport. Impedance heaters often break down when exposed to these loads, resulting in failure of the heating system. Electrical or dielectric insulation typically is positioned between the inner pipe and the outer pipe to electrically isolate the pipes. Gaps often exist between the inner pipe and the dielectric insulating material because of the nature of the materials commonly used, and/or how the materials commonly are attached. Gaps also are necessary to allow for differences in the rates of thermal expansion between the inner pipe and the dielectric insulator. When loads accelerate across these gaps, coming to an abrupt stop at the other side of the gap, a significant hammering action results which often is sufficient to break or shatter a rigid insulating material such as ceramic. Therefore, it is desirable to develop a support structure for an impedance heating system that is able to withstand the forces associated with rail car transportation, and prevents insulation failure while allowing for thermal expansion of the system. It is further desirable to use an insulating material with high compression capabilities that will not break when subjected to high forces.
Drawbacks associated with impedance heating systems used for pipeline storage and distribution systems include thermal insulation failure and accidental grounding of the system. The pipeline impedance heating system typically includes a thick layer of rigid insulation such as calcium silicate that is wrapped around the inner pipe to thermally insulate the pipe and prevent heat loss. An outer jacket, often constructed from a material such as aluminum sheet metal, is then wrapped around the insulation to protect the insulation from the environment. These thermally insulated pipelines can be up to twelve inches in diameter and several hundred feet in length, with bends, turns, rises and drops, all of which require substantial structural support. The structural support must be capable of safely supporting the insulated pipeline from expansion and seismic loads imposed from various directions. Since an electrical potential is imposed on the pipeline, the supports must be capable of not only structural support, but also of maintaining dielectric isolation of the pipeline from ground. The outer jacket material of pipeline impedance heating systems degrades over time as the assembly shifts or moves as a result of thermal expansion and contraction of the pipeline, pumping loads and vibrations such as earthquakes. If the integrity of the outerjacket is compromised (e.g., torn or punctured), the thermal insulation may become damp or damaged when exposed to certain environmental conditions, thereby resulting in loss of the dielectric properties as well as the thermal insulating properties of the material, further resulting in the grounding out of the system and system failure. The insulation also may be damaged upon assembly of the system. Therefore, it is desirable to provide a support structure for pipeline systems that will not cause system failure should the outer jacket or thick insulation wrapped around the inner pipe become damaged.
Such a pipeline assembly typically is supported by a foot connected to the inner pipe and sitting on a structural base with a dielectric insulating material between the base of the foot and the structural base (electrical ground). The dielectric insulating material may degrade over time because of environmental and structural load conditions, and thus "short-out" the system. In this configuration, the heating system can be short circuited when the insulation or support foot degrades over time or because of environmental conditions. When a short occurs in the impedance heating system, it can be difficult to locate the problem area (e.g., between the support foot and ground, or between a jacket bracket and the support foot). Often, the system must be disassembled and all the insulation must be removed from the inner pipe to find the source of the problem. Therefore, it is desirable that the support structure used to protect and insulate the inner pipe be adaptable to provide the necessary support for a pipeline system that will not pose a risk of short circuiting the system when used for an extended period of time or in harsh environmental conditions.